In e.g. line interface board applications, supply voltages are generated to subscriber line interface circuits on line interface boards. These supply voltages are generated by means of DC/DC converters from a DC supply voltage, normally -48V. The voltages that have to be generated by those DC/DC converters are normally between -20V and -120V depending on the application.
If voltages are to be generated between ground and the DC supply voltage, a so called buck regulator is suitably used. For voltages below the DC supply voltage, a so called boost regulator is used.
In some applications, voltages above as well as below the supply voltage are to be generated by means of the same regulator. In such cases, a combined buck boost switching regulator is needed.
An example of a known buck boost switching regulator is disclosed in U.S. Pat. No. 4,578,630.
That known buck boost switching regulator has two control loops which may cause stability problems. Moreover, suppression of disturbances in the form of AC components on the DC supply voltage is poor in the known regulator. Thus, the known regulator would be unsuitable for use in e.g. line interface board applications.